20g is definitely big enough for a pair. I'm keeping a pair of A. borellii in a planted 20g with a few dithers (will remove them once the A. borellii get more confident). Here is a video of it. You can see the female in the first few seconds.
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I also have a group of 6 A. rotpunkt breeding in another 20g. 5 males and 1 female I think....miraculously they are getting along, and the female breeds with the dominant male every couple of weeks.
Also tried out over crowding Apistos recently and it's been working very well too - I have probably two dozen individuals (4 species) in a planted 90g with some young Geophagus and angels, and there's no fighting whatsoever. I think if I remove other fish, I can easily add a few more dozens without any problem.